Comparisons with Previous Solar Lighting Systems

Cost and Economic Value

Researchers at the University of Wisconsin–Madison’s Solar Energy Laboratory recently completed a break-even capital cost analysis comparing hybrid configurations with traditional lighting. They assumed an installed system cost of between $3000 and $4000 for a commercial system capable of collecting and distributing 2m² of sunlight. Model results indicated that the added cost of a hybrid system could pay for itself in well under 5 years. This assumed that the system was installed in a building located in the Sunbelt, and that the hybrid configuration included the use of incandescent or halogen lamps in task-lighting applications where accurate color rendering is important.

Table 3 compares the projected cost and performance of the proposed system with that of a state-of-the-art commercial system. Accordingly, the anticipated cost per delivered lumen of the proposed system far exceeds its only commercial counterpart.

Cost considerations

The concept for hybrid lighting has been around since the early 1970s, but it has been difficult to make the technology practical. Japanese researchers previously developed solar collectors with glass optical fibers — which are more heat-resistant, but also more expensive and harder to work with. According to Dr. Duncan Earl, ORNL, the glass set-up cost around $40,000 to illuminate 1,000 square feet. Earl and his collaborators have cut down on costs by using plastic optical fibers — as well as plastic mirrors. They are still in the prototype stage, but they hope to get the price to $3,000 for 1,000 square feet. If they can reach their goal, a building owner in Hawaii could pay for implement-ing the new technology in 2-3 years, with the savings on electricity bills. In other parts of the country, where sunlight is reduced and utility costs are less, this payback would take longer.

The payback period of hybrid solar lighting lengthens in proportion to the efficiency of the electric lamps used in combination with distributed sunlight. Because linear fluo-rescent lamps are very efficient (80–95 lum/W), the models indicate the additional cost of a hybrid configuration used with such lamps will require more than 10 years to pay for itself in most regions of the country during the early years of commercialization. As costs fall, HSL has the potential to become cost-competitive in a majority of indoor lighting scenarios.

The Wisconsin study also concluded that a hybrid configuration would likely extend the typical life of incandescent and/or halogen lamps from 2000 to 8000 hours. When in-candescent lamps are dimmed, filament temperatures decrease. As filament tempera-tures decrease, life expectancy increases. Although the lamps will last longer, a penalty in efficiency occurs because cooler filaments are generally less efficient at radiating visible light.

In contrast to skylights, roof penetrations for HSL are few and small, reducing the potential for leaks. As R&D improves system performance, increases system lifetime, and reduces system cost, greater penetration into the larger market of existing buildings and commercial buildings with fluorescent lighting only will be possible. As system price is reduced and secondary benefits of the technology are demonstrated (e.g., immproved employee productivity), using HSL with large building spaces using all fluorescent lighting will be more cost effective. The levelized energy costs at that stage are expected to be approximately 9 cents/kWh (see below):

• Capital cost: $4k
• Project lifetime: 30 years
• Discount/interest rate: 8%
• Annual O&M cost: $200
• Annual energy savings: 5700 kWh
• Capital recovery factor: 0.089
• Levelized energy cost: $0.097/kWh

*Equates to <$100/kilolumen, which is below entry point for SSL in same first market
**Minimal investment to date

Comparison with alternatives in buildings

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Last Revision: July 2005